Alcator C-Mod Weekly Highlights April 19, 2004 Research operations continued at Alcator C-Mod last week. A total of 65 Research Days have now been accomplished during the current fiscal year, amounting to 90% of the operational target of 18 weeks (72 days) specified in JOULE Milestone SC7-6a. Experiments were carried out in support of MHD, Transport, Devertor/Edge, and diagnostic development research, including two days devoted to ITPA Joint Experiments. Progress also continued on the preparations for the Lower Hybrid system. Research operations are scheduled to continue this week. Operations ---------- Four run days (Tuesday - Friday) were scheduled and completed last week. A total of 110 plasma discharges were produced with a startup reliability of about 90%. A fresh boronization was carried out on Thursday night in preparation for the experiment on Friday. Tuesday's run was devoted to MP#361, the ITPA Joint Experiment on locked mode thresholds with dimensionlessly identical plasma parameters on JET, C-Mod and DIII-D. As in the run carried out in the previous week, the high disruptivity inherent to the experiment limited the amount of data obtained. All discharges were carried out in the "JET shape" with Bt=6.3T and Ip=1.3MA, using the C-Mod A-coils in a configuration which closely matches the mode spectrum of the JET EFCC coilset; both polarities of A-coil current were used to induce locked modes. Further analysis is now needed to attempt to remove the effect of the intrinsic error variation during these discharges. In terms of further runs, data with nebar>4e20/m^3 is needed, as these data are expected to be most insensitive to the intrinsic error field. Also more complete data is needed with positive A-coil currents and nebar~2.4e20. Finally data in an alternate A-coil configuration with slightly differnt mode spectrum and toroidal phase is desirable. On Wednesday, most of the run was devoted to MP#393, "Magnetic fluctuations associated with edge plasma turbulence". The two-coil magnetic probe head installed on the A-port drive (ASP) was used to record poloidal magnetic field fluctuations (frequency and k-poloidal resolved) as a function of depth in the scrape-off layer. The experiment was designed to investigate changes in the character of magnetic fluctuations in ohmic L-mode as the edge plasma collisionality is changed. A density scan from nebar~0.5e20 to 3e20/m^3 was carried out in 800kA, 5.4T, lower single null discharges. In addition, comparison shots over a similar range of density were obtained in upper single null discharges. The magnetic data clearly showed a reduced poloidal phase velocity (i.e., not as strong in the electron diamagnetic direction) in these discharges compared to similar ones in lower single null. The goals of MP#393 were completed in about six hours on Wednesday, and the remainder of the run was spent obtaining additional data for MP#386, on locked mode thresholds. Additional data at 1MA, 5.4T and low density (nebar<5e19/m^3) was obtained. These data are being analyzed. On Thursday, we carried out experiments to test the performance of the Compact Neutral Particle Analyzer (CNPA) diagnostic (MP#389). This diagnostic, which is being developed as part of a graduate student thesis project, employs a compact silicon diode to observe minority tail ions produced by ICRF, which are converted to neutrals by charge exchange with the diagnostic neutral beam (DNB). Pulse height analysis is used to measure the ion energy distribution. The experiment was carried out in L-mode at nebar<7e19/m^3 to maximize the fast ion tail. A scan of RF power below the H-mode threshold, around 1.3MW for these conditions, was carried out, along with a scan of toroidal field to examine the effect of changing the resonance location. Finally a density scan, up to nebar~1.4e20/m^3 was carried out at fixed field and power; neutral energy and count rate was reduced, as expected, as the density increased. More detailed analysis is in progress. Friday's run was devoted to MP#397, an ITPA Joint Experiment to assess the similarities and differences of the C-Mod EDA regime with JFT2M HRS regime, in particular the fluctuation properties and access regime. The experiments were carried out in the "JFT-2M shape" (except for aspect ratio), elongation~1.5, delta_u~.2, delta_l~.8, which had been developed in an earlier run. Most of the run was spent scanning power and density at q95=3.5, in order to vary the collisionality. At most densities, a steady EDA regime was seen. However, fluctuation properties on PCI and magnetics (using probe in dwell mode) showed two distinct frequency peaks, which is not typical for C-Mod EDA. At the lowest densities, the discharges entered what appears to be a regime with large distinct ELMs and no QC mode. Experiments were also carried out at a lower q95~2.9, for which JFT2M was recently able to access HRS regime at higher collisionality. As is typical for C-mod at this q, the H-modes were no longer steady but consisted of several short H-modes with back transitions. There were some edge fluctuations, which need to be examined, but they were rather erratic and incoherent. At higher density, the H-modes did become longer and steadier. We obtained a range of plasma conditions at medium q which will be worthwhile to evaluate and compare in detail with the results of JFT-2M. The run did produce some fluctuations which were different from those we have been seen before, and at least qualitatively similar to those on JFT-2M. An interesting result was that this shape tended to produce ohmic H-modes much more easily than our standard shape. Many of the discharges, at both q values, had their first L-H transitions well before the RF turned on. This in fact occurred more at the higher densities,contrary to usual expectations, based on experience in discharges with higher elongation and more balanced upper and lower triangularity. Physics -------- An initial survey of results collected during 2-1/2 run days dedicated to MP#384 has been performed ("Experiments on Plasma Flows and the L-H Threshold: low density discharges, limited versus diverted topologies and the role of neutrals"). In particular, the influence of four different magnetic topologies on the L-H power threshold was examined: (1) standard lower single-null, (2) lower single-null with separatrix just grazing the inner divertor nose, (3) discharge limited on inner divertor nose tiles, and (4) discharge limited on inner midplane tiles. No difference in L-H threshold power was observed for cases (1), (2) and (3), spanning the line-integral plasma density range of 0.7 to 1.0x10^20 m^-2. Remarkably, regardless of being a lower single-null diverted or lower divertor "nose-limited" discharge, these plasmas consistently exhibited an L-H transition at about 1.7 MW of total (RF plus ohmic) input power. In contrast, a dramatic increase in power threshold was observed as the gap to the inner wall tiles was reduced in lower single-null discharges: A gap of 1.5 mm required over 3 MW of total power; an L-H transition for an inner-wall limited discharge was seen, but only at or above the total input power level of 4.9 MW. These results contribute to JOULE Milestone SC6-1b "Compare energy confinement, H-mode thresholds, and divertor particle dynamics in Alcator C-Mod discharges". ICRF System ----------- The ICRF System performed nominally in support of experiments on Thursday and Friday. During testing on Wednesday, FMIT#1 was found to be inoperative. The origin of this difficulty was traced to the low power 80.5 MHz signal source, which had apparently "latched up" as a result of an AC line power glitch. Power cycling the unit restored proper operation. The two FPA tubes previously removed from FMIT#1 and 4 were received at the vendor for evaluation. Lower Hybrid System ------------------- Doug Loesser, Joe Frangipani, and Dave Miller, all from PPPL, visited last week to work with MIT personnel on the forward to rear waveguide assembly. The components were aligned and mated without the rf gasket. Measurements required to complete fabrication of the final support plate were then made. Assembly of the unit with the rf gasket and support plate will continue this week in collaboration with PPPL. We are prepared to begin high power tests of the unit as soon as assembly is complete. Work continued on procurement of the alumina windows needed to complete brazing of the lower hybrid couplers. Alumina test coupons have been metalized with MoMn by our vendor as a qualification step before proceeding with the actual windows. We continued to discuss details of the Ni strike and Cu plating with the vendors, as we attempt to understand why some of the previous batch of windows did not plate properly. Several window samples have been sent out for analysis. We continued development of the low power microwave control system. Discussion with one of the software vendors should soon result in improvements to one of the real-time drivers used by the phase and amplitude control program. We investigated nonlinear behaviour in the I/Q detectors needed to measure the phase of the launched rf waves, and we developed more calibration software for the control loop. Travel and Visitors -------------------- Tim Hender (UKAEA, JET) completed his visit to C-Mod to participate in Joint Experiments on error fields and locked modes; he served as Session Leader for Tuesday's experiment. Doug Loesser, Joe Frangipani and Dave Miller (PPPL) were at MIT 4/13-14 to work on the assembly of the Forward Waveguide to the Rear Waveguide of the C-Mod Lower Hybrid Launcher. Martha Redi was at MIT this week, 4/12-16. She worked with John Rice and Catherine Fiore on analysis of off-axis RF plasma transport in preparation for the upcoming TTF and EPS meetings. Prof. Ambrogio Fasoli of the CRPP/EPFL visited the PSFC from 13 - 16 April to participate in piggy back experiments using the Active MHD antennas to search for TAE resonances. Together with a great deal of help from the C-Mod engineering staff, we managed to get one of the high frequency Active MHD amplfiers working in sweep mode to obtain a series of TAE resonances in the plasma with resonant frequencies in the range of 300 kHz by sweeping the antenna frequency from about 250 to 350 kHz. Damping rates obtained with relatively large outer gaps (> 1 cm) in diverted plasmas were in the range of 4.5%. Earl Marmar and Amanda Hubbard attended a meeting of the FESAC Priorities Panel, held in Chicago on Monday, April 12.